netfilter: nf_tables_offload: check FLOW_DISSECTOR_KEY_BASIC in VLAN transfer logic

The VLAN transfer logic should actually check for
FLOW_DISSECTOR_KEY_BASIC, not FLOW_DISSECTOR_KEY_CONTROL. Moreover, do
not fallback to case 2) .n_proto is set to 802.1q or 802.1ad, if
FLOW_DISSECTOR_KEY_BASIC is unset.

Fixes: 783003f3 ("netfilter: nftables_offload: special ethertype handling for VLAN")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>